That's a tiny issue that can be shown only if you force the custom region to have a transparent background on resizing (f.e. Using the snapshot region). If you reduce the region size, the canvas content under the region (as z-index) is shown, but if you enlarge the region the canvas content disappears.
Best regards,
Davide
MOVED IN THE BUG TRACKER
If you like my plugins please consider to support the program buying a license; for personal contributions to me: paypal.me/dcprojects